pki_crypto: plug pki_signature_from_blob leaks

In 3341f49a49a07cbce003e487ef24a2042e800f01, some direct assignments
to OpenSSL structures was replaced with usage of getter and setter
macros. Ensure to bignum_safe_free a couple of intermediate values
in error paths for pki_signature_from_blob DSS and ECDSA cases.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>